These are the final days of the BBC's unloved digital channels, Choice and Knowledge. Their mercy killing was decreed by Greg Dyke soon after he became director general, although he hid the news of their execution within his announcement of the launch of BBC3 and BBC4. Supplemented by two new children's services to occupy the daylight hours, BBC3 will soon replace the Choice that so few chose, and BBC4 the Knowledge that no one wanted to know.

I say replace. In fact, it astounds me that the BBC felt it necessary to go back to the government for permission to tweak and rechristen. Tessa Jowell, the Secretary of State for Culture, Media and Sport, called its bluff and, after complaints from commercial broadcasters, asked it to make BBC3 "more distinctive". Never mind, BBC4 "launches" on 2 March and BBC3 will surely follow: its boss, Stuart Murphy, already has an office between those for the controllers of BBC2 and BBC4.

As for "distinctive", the first thing BBC3 must be distinctive from is BBC Choice, which failed as a catch-up station of repeats and is now failing as a youth channel. I doubt, however, if viewers will notice much change. BBC Choice is already using the BBC3 logo of three spinning cubes, already has a blink-or-you'll-miss-it news bulletin, and already has its basic scheduling blocks in place: Liquid News at 7pm, and the Johnny Vaughan chat show at 9pm. The controller of Choice is, incidentally, Stuart Murphy.

If all BBC3 turns out to be is BBC Choice renamed, it is in trouble. Whereas there may be a premium in E4 showing The Sopranos and ER a few months before Channel 4 does, it is hard to believe that there are many Choice viewers who could not have waited for Vic and Bob's Shooting Stars to make it to BBC2 in a few weeks' time. And many of its home-grown programmes are poor. Choice's current idea of post-pub infotainment is a documentary series called Toilets (Mondays, 11pm). I would like to have been in the ideas meeting that came up with this one; I doubt if it lasted very long.

Winston Churchill reckoned "Please adjust your dress before leaving" to be one of the universal cliches, but this programme contained every cliche in the J Street-Porter handbook on how to address yoof. In Claudia Winkleman, we had the flirtatious, naughty-but-nice yoof presenter. We had bits of speeded-up film, off-kilter camera angles, irrelevant fact boxes and much tittering smut. The 28 January episode actually demonstrated the positions in which to have sex in public lavatories. The only difference between it and scores of such programmes knocked out by Bravo or Granada Men and Motors was its budget, which was substantial enough to take Claudia to the very loo in Beverly Hills where George Michael had been arrested for performing "a lewd act". As Winkleman said: "My parents are so proud . . . "

Meanwhile, BBC4 launches for certain on 2 March. Its controller is Roly Keating, who, uncoincidentally, has been in charge of BBC digital channels for a year. He has an annual budget of £35m - tiny in BBC terms, but £28m more than his predecessor, Liz Cleaver, had at Knowledge. The first card he will play is an upmarket bulletin of world news at 8pm every evening. The mere introduction of something live into the mix will make the station look less archival. This will help considerably, because the problem with archives is that you feel, as they will always be there, that you can put off your next visit indefinitely.

As its little sister was showing Toilets, BBC Knowledge offered us Storyville: Baria's big wedding (10.35pm), a rather sad documentary by Manu Bonmariage, about a teenager from Marseilles whose ridiculous mother, a citizen of the Federal Islamic Republic of the Comoros, had promised Baria's hand to a passing Comoran boy before she was even born. With one failed marriage already behind him, her 34-year-old fiance, Maoulida, could not believe his luck.

At first, I did not understand how I had missed this film when it was first transmitted on BBC2. The Storyville strand of foreign documentaries is always worth keeping an eye on. I then discovered that it had in fact been shown on BBC Knowledge itself. Although there is nothing wrong with repeats on a digital channel (a BBC Knowledge documentary is lucky to get 50,000 viewers on a single outing), it was a jolt to realise that this edition of Storyville had been immediately bumped off to the digital hereafter. Indeed, most Storyvilles no longer appear on BBC2.

Under Keating, BBC4, buried deep in the SkyGuide, will continue to suffer from a shortage of viewers. But suppose the critics discover it and it becomes their darling. What fury will that induce in non-digital viewers? How will they see the Storyvilles they read about? Who will explain to them that The Trial of DalI, starring Stephen Fry and Vic Reeves, is now thought too challenging for BBC2? Meanwhile, to succeed with its target audience, BBC3 must not look like a public service broadcaster at all, which will anger the purists in parliament and the commercial lobby equally.

At the moment, BBCs Choice and Knowledge are safely hidden embarrassments. We must admire Dyke for coming clean and making them costly and visible whipping-boys.

Andrew Billen is a staff writer on the London Evening Standard